i have seen people say as little as .25 inch up to 1 or more inches. i usually am on the lesser side with my mya mini since the pull is choppier since the base is small

as to foil, regular foil most people use 2 layers, the heavy duty usually one, heavy strength is supposedly the rename but i think they are different thickness, if you burning or getting a lot of harshness then switch to 2 layers